Azul,
Definitely consult a pediatrician and/or pharmacist for guidance here. Generally, for kids, the dosages have to do with age and weight. With age, up until the teens, it's half the adult dosage. For teenagers, depending on weight, it's usually 3/4 of the adult dosage. The weight element is used to simply make the dosage as exact as possible. Plus, girl, with some of the teens I see, saying they look like adults is an understatement.
With me, my mom started me on a potent multivitamin when I was around 10 and she also had me take Vitamin C. When I got to be a teenager, I started with a B-complex.
You might do some Internet browsing via Google for Children's Vitamins or Teen Vitamins. I know I have seen two companies at least which make hair vitamins specifically for teens.
I know that you are helping your children to eat good nutritious meals and snacks which is really a gift beyond description because that's for a lifetime. With any vitamins, these, as we mention here at the site, are always an adjunct to sound nutrition.
Much Love,
Jade